This proposal has two main objectives: a) to determine whether changing from one gait to another is a mechanism for maximizing muscular efficiency as running animals increase speed; and b) to determine the relationship between muscular activity and basal metabolism by studying the effect of equalizing daily activity in species with widely divergent basal metabolic rates. During the second year of this study, we propose to complete electromyographic measurements on 18 limb and trunk muscles of the dog and a second quadruped (horse or pig) to determine how the activity of muscles change as a function of gait and speed. To date, we have found no discontinuities in the firing patterns of any of the limb muscles as a function of speed when dogs change gait. We have found, however, that the major trunk muscles are recruited when animals begin to gallop. We have also found a discontinuity in the recruitment of low oxidative fibers in the triceps and gluteus medialis of lions as they changed from a fast trot to a slow gallop by using glycogen depletion and biochemical analyses of muscle biopsies. We propose to extend these studies to a second quadruped. We proposed to measure oxygen consumption of whole animals over 48-hour periods, metabolic enzyme potential and protein turnover in muscle, and muscle substrate resynthesis in hedgehogs, lizards and rats during normal activity and during conditions where activity is equalized. These animals have basal metabolic rates which normally differ by more than twenty-fold and similar incremental energy expenditures for equivalent locomotion.